\documentclass[11pt]{article}
    \usepackage[utf8]{inputenc}
    \usepackage{booktabs}
    \usepackage{array}
    \usepackage{siunitx}
    \usepackage{tabu}
    \usepackage{longtable}
    \usepackage{makecell}
    \usepackage{multirow}
\begin{document}
    \begin{table}
        \centering
        \begin{tabu} to 1.1\textwidth{| X[l] | X[l] | X[l] | X[l] | X[c] | X[c] |}
            \cline{2-6}
            \multicolumn{1}{c|}{} & \multicolumn{3}{c|}{Voltage measured (V)} & \multirow{2}{*}{\thead{Mean\\ voltage (V)}} & \multirow{2}{*}{\thead{Standard\\ deviation}}\\
            \cline{1-4}
            Temperature (\SI{}{\degree}C) & Replicate 1 & Replicate 2 & Replicate 3 & &\\
            \hline
            10 & 1.79 & 1.78 & 1.79 & $\frac{1.79+1.78+1.79}{3}=1.78$ & 0.00577\\
            \hline
        \end{tabu}
    \end{table}
\end{document}
